The inverse ratio of 25:1 is 1:25.
That's one way to look at this problem. It might also be asking you for the same ratio expressed in terms of 1:something or 1/something rather than 25 to something. In other words, if there are 2 pencils per child (2:1) then there are 1 pencils per 1/2 child (1:1/2).
Divide both 25 and 1 by 25. 25/25 : 1/25 is 1:1/25 or 1:.04.
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